This invention relates to semiconductor junctions and more particularly to semiconductor devices having P-N junctions having a high avalanche breakdown voltage characteristic. The semiconductor devices are respectively comprised of a semiconductor substrate having impurity atoms of one type and having a first surface. A doped region lies in the substrate at the first surface and has dopant impurity atoms of a type opposite to the impurity atoms of the semiconductor substrate. For example, the substrate may have P-type impurity atoms, such as boron; while the doped region may be formed in the P-type substrate by N-type impurity atoms, such as phosphorus, arsenic or antimony. Typically, the doped region extends from the first surface into the substrate to one uniform depth of approximately 0.1-5.0u. The perimeter (i.e. sides) of the doped region extends from the first surface to the bottom of the doped region and has a curvature. This curvature is roughly approximated by a fixed radius. The curvature is important because the curved geometry causes electric field lines to crowd at the perimeter. Under a high reverse bias voltage, the electric field line crowding gives rise to an avalanche breakdown voltage that is lower than the breakdown voltage of the portion of the doped region with the uniform depth.
The concept of avalanche breakdown, and the operation of the invention to increase the bias voltage at which avalanche breakdown occurs, is best understood by considering the dynamics of the mobile and immobile charge carriers at the junction under several different bias voltage conditions. With zero reverse bias applied to the junction, each side of the junction has a large number of mobile or free charge carriers. In particular, mobile electrons exist in the conduction band in the N-type material; while mobile holes exist in the valence band in the P-type material. In addition, each side of the junction has a large number of immobile charge carriers which provide a fixed space charge. This space charge is comprised of positive ions in the N-type material, and negative ions in the P-type material.
Also under a zero bias voltage condition, a depletion region exists at the junction. This depletion region is formed by free electrons drifting from the N-type side to the P-type side where they recombine with holes, and by holes drifting from the P-type side to N-type side where they recombine with free electrons. The net effect of this mobile charge annihilation is that both sides of the junction are depleted of free charge. At the same time, the positive immobile charge in the N-type side and the negative immobile charge in the P-type side remain. The area in which this immobile charge exists with no cancelling mobile charge is called the depletion region.
The immobile charge in the depletion region gives rise to an electric field across the junction; and large electric fields cause avalanche breakdown. In particular, the electric field accelerates electrons as they cross from the P-type material to the N-type material. These accelerated electrons collide with other atoms as they pass through the junction. If the accelerating electric field is sufficiently high, the collision ionizes the atom. That is, the collision generates another free electron in the conduction band and a mobile hole in the valence band. The new free electron and the original free electron are then both accelerated and other similar collisions result. The electric field required to produce sufficient acceleration to cause ionization, is called the critical field; and it is approximately (2-8).times.10.sup.5 volts per centimeter in an abrupt junction having a substrate doping of 1.0.times.10.sup.15 atoms/cm.sup.3.
In general, the electric field at any point in the junction is a function of the reverse bias applied to the junction and the junction geometry. A large reverse bias applied to the junction increases the depletion region width, and the size of the electric field increases accordingly. Where the junction is relatively flat, the electric field lines are approximately perpendicular to the junction and thus are parallel to each other. But where the junction has curvature, the electric field lines are also approximately perpendicular to a junction and thus they crowd on the concave side of the junction. Accordingly, the electric field is highest where the junction curvature is greatest. It is in that region that the critical electric field is first reached as the reverse bias is increased, and avalanche breakdown first occurs. A more detailed analysis of electric field line crowding and its effect on avalanche breakdown is described in an article entitled Breakdown Voltage of Cylindrical Gaussian P-N Junctions by Rombeek et al printed in the Solid State Electronics Journal, volume 24, pgs. 1193-1200, June 1971.
The disclosed semiconductor device has a P-N junction provided with a novel structure which greatly reduces electrical field crowding at the perimeter of the doped region. As a result, the disclosed structure has a high avalanche breakdown voltage characteristic. In the prior art, several other structures are described which also reduce electric field crowding at the perimeter of the doped region. But these structures are completely different from the disclosed structure. For example, an article entitled Edge Breakdown in Mesa Diodes by Kelholm et al published in the IEEE Transactions on Electronic Devices, volume 18, No. 10, pgs 844-848, October 1971 describes the structure and operation of a mesa diode. The mesa diode in general, reduces electric field crowding at the perimeter of the doped region by removing a portion of the substrate adjacent to the perimeter of the doped region. For example, the substrate adjacent the perimeter of the doped region is removed by an etching treatment after the doped region is formed.
Another junction structure having a high avalanche breakdown voltage is described in an article entitled The Theory and Application of a Simple Etch Contour for Near Ideal Breakdown Voltage in Plane and Planar P-N Junctions by Temple et al published in the IEEE Tranactions on Electronic Devices volume 23, No. 8, pgs. 950-955 August 1976. The structure therein disclosed achieves increased avalanche breakdown voltage by having a doped region which is etched near its perimeter. That is, the doped region has one thickness near its perimeter and has a greater thickness at its interior.
Still another semiconductor junction having a high avalanche breakdown voltage is described in an article entitled New Semiconductor Devices of Ultra High Breakdown Voltage by Matsushita et al which was published in a technical paper in the International Electron Devices Meeting, pgs. 109-110, in December of 1973. The structure therein described is basically comprised of a semiconductor substrate having a doped region lying therein forming a conventional PN junction, and further including a plurality of doped regions which are spaced apart and surround the first doped region. In the illustrated embodiment, the plurality of doped regions are circular in shape and form concentric rings about a circular conventional doped region.
Still another structure for a junction having a high breakdown voltage is described in an article entitled Effect of Surface Fields on the Breakdown Voltage of Planar or Silicon P-N Junctions by Grove et al printed in the IEEE Tranactions on Electronic Devices volume 14, No. 3, pgs 157-162, March 1967. The structure therein disclosed includes a semiconductor substrate having a doped region lying therein, and further includes a gate electrode which surrounds the perimeter of the doped region. A gate voltage is applied to the gate electrode to thereby generate an electric field in the substrate near the perimeter of the doped region. This generated field counteracts the electric field due to the depletion region and thus reduces field crowding at the perimeter.
However, all of these prior art structures are totally different than the disclosed structure. It is therefore an object of the invention to provide a new and non-obvious semiconductor junction structure having a high avalanche breakdown voltage.